Monday, August 08, 2005

Legal Alternate: Part Two

A few days ago I began explaining the minimum weather requirements for an IFR alternate on a Canadian flight plan. See if you can remember what 400-1, 600-2, and 800-2 mean. Here's how to apply them.

Michael Oxner, who has a scanner and knows how to use it, has some approach plates online with beautiful explanations. I'll use his Moncton ILS or NDB RWY 29 (that will open in a separate window, if I did it right) as an example. The top right hand corner of the plate tells us the aerodrome elevation (the height of the highest point on the runways) and the touchdown zone (the first third or first 3000' of the runway, whichever is shorter) elevation for runway 29. They're both the same, at 232', which implies that the airport is flat. I like that in an airport.

Down near the bottom of the plate, opposite the category ILS you see 432 (200). The altitude 432' is the decision height while the (200) informs us that it's 200' above the touchdown zone, saving us from doing the arithmetic to subtract 232 from 432. The 1/2 RVR26 is the advisory visibility: 1/2 mile or RVR of 2600 feet, same thing.

To determine the required weather to file Moncton as an alternate, lets say that the runway 29 ILS is the only useable precision approach at the airport. That might be because there was no ILS for the runways 06/24, because the forecast wind was unsuitable for landing on any other runway, or because the other runway was unavailable for some other reason. So Moncton would have one useable precision approach. That requires weather of 600-2 or 300' and 1 above minima, whichever is greater. Adding 300 + 200 gives 500, and 1 + 1/2 = 1 1/2, so the standard 600-2 is greater, and applies. The forecast ceiling must be at least 600 feet above ground, and the forecast visibility two miles for this airport to be the alternate.

At St. Anthony, Newfoundland depicted at the bottom of the page, there are no precision approaches available, so we need 800-2 to use this as our alternate. But the MDA of 600' is 508 feet above the aerodrome. Adding 300 + 508 = 808, and 1 + 1 1/2 = 2 1/2, so the minimum weather is greater than 800-2.

Weather at the airport is always forecast to the nearest hundred feet, so when you need 808 feet, does that mean you need 800 or 900 feet? The changeover altitude is 20', meaning that a calculated required ceiling of 808 feet requires a forecast of 800', while a calculated requirement of 821 feet requires of at least 900'. Calculated visibility requirements of over 3 sm require no more than 3 sm forecast vis. There aren't many cases this pathological, but you can bet the guy who does your next PPC ride knows of one in your approach plate book.

I still didn't get to airports with GPS approaches or no approaches. There's so much in this tiny corner.

1 comment:

Elaine Morrison said...

Is the max calculated alternate advisory vis still 3sm in 2018? I've seen this in my Harv's Air IFR course and the 2017 Culhane exam book but a 2018 Canadian Instrument Pilot exam book q 53 has it wrong (they posted 4sm as correct). There was only 2 or 4sm available in their question with incorrect TAF ceilings for the other options. I'm convinced the 4sm is wrong and it is still max 3 which is MVFR, visual approach, and non-IFR approach alternate minima anyway, so demanding any more would logically be overkill, especially when in a pinch you are going to shoot to land in much lower advisory visibility anyway.